Ion mobility spectrometry is a versatile technique used to detect presence of molecular species in a gas sample. The technique has particular application in detection of explosives, drugs, and chemical agents in a sample, although it is not limited to these applications. Portable detectors are commonly used for security screening, and in the defence industry.
Ion mobility spectrometry relies on the differential movement of different ion species through an electric field to a detector; by appropriate selection of the parameters of the electric field, ions having differing properties will reach the detector at differing times, if at all. Time of flight (TOF) ion mobility spectrometry measures the time taken by ions when subject to an electric field to travel along a drift tube to a detector against a drift gas flow. By varying the electric field ions of different characteristics will reach the detector at different times, and the composition of a sample can be analysed. This form of spectrometry relies on the length of the drift tube for its resolution; the longer the drift tube, the more powerful the detector.
A variation on TOF ion mobility spectrometry is described in U.S. Pat. No. 5,789,745, which makes use of a moving electrical potential to move ions against a drift gas flow towards a detector. A plurality of spaced electrodes are alternately pulsed to generate a moving potential well, which carries selected ions along with it.
Field asymmetric ion mobility spectrometry (FAIMS) is a derivative of time of flight ion mobility spectrometry (TOFIMS), which potentially offers a smaller form factor; however, existing designs use moving gas flows and high voltages, which are undesirable for microchip implementations. Scaling is further hindered by molecular diffusion, an effect that becomes significant in the micron regime. Background information relating to FAIMs can be found in L. A. Buryakov et al. Int. J. Mass. Spectrom. Ion Process. 128 (1993) 143; and E. V. Krylov et al. Int. J. Mass. Spectrom. Ion Process. 225 (2003) 39-51; hereby incorporated by reference.
A further modification of FAIMS is described in international patent publications WO2006/013396 and WO2006/046077, the contents of which are incorporated herein by reference. The devices described in these publications make use of an electric field to cause ions to move toward the detector, and an ion filter comprising paired interdigitated electrode structures defining a plurality of ion channels through which ions may selectively pass, depending on the electric field applied between the electrodes. The paired electrodes must be electrically separated from one another; this is achieved by mechanical separation of the electrodes by forming them on an insulating substrate. To manufacture these electrodes, they must either be separately manufactured and then bonded onto a substrate, or they must be formed directly on the substrate.
As described in WO2006/013396, the electrodes may be manufactured using largely conventional microfabrication techniques. A conductive material is deposited on the top and bottom faces of a high resistivity silicon wafer substrate, followed by a photo resistant coating on each face. The top face is masked and subjected to photolithography, after which the coating of the top face is wet etched to provide an array of electrodes. The photoresist is stripped from both faces, and the process repeated to form the bottom face electrodes. A further resist coating is applied to the top face, after which the silicon is etched from the lower face using deep reactive ion etching to form channels. The photoresist is stripped for the final time, and the filter is ready for further processing.
In a variation of this technique, the silicon wafer may be initially bonded on the bottom face to a glass substrate; the various etching steps are then carried out from the top face to create channels and electrodes, after which the glass substrate is acid etched to expose the bottom face of the wafer, leaving a glass support in contact with the wafer.
The ion filter structure so produced may then be used in an ion mobility spectrometer device; or may be used in an ion pump, as described in GB 0521451.5.
A monolithic quadrupole mass spectrometer is described in GB 2 391 694, while US 2001/0042826 describes an ion filter including a two-dimensional array of poles forming one or more quadrupoles.
It would be advantageous to provide an alternative means of manufacturing an ion filter. In particular, it would be advantageous to reduce or remove the need for an insulative substrate.